In the field of mobile communication systems (for example, personal handyphone system: PHS) evolving rapidly these few years, an adaptive array base station is adapted to practical usage. The adaptive array base station separates and extracts a signal of the desired wave by applying the well known adaptive array processing on the reception signal of an array antenna composed of a plurality of antennas in order to suppress the effect of interfering waves to obtain favorable communication quality.
Furthermore, by employing such an adaptive array base station, a PDMA (Path Division Multiple Access) system can be realized. The PDMA system allows mobile terminal devices of a plurality of users to be subjected to path division multiple access to a radio base system by dividing the same time slot of the same frequency spatially in order to improve the usage efficiency of radio frequency. The PDMA system is also called the SDMA system (Space Division Multiple Access) system.
FIG. 11 represents the channel arrangement of the various communication systems of frequency division multiple access (FDMA), time division multiple access (TDMA), and space division multiple access (SDMA).
First, FDMA, TDMA and SDMA will be described briefly with reference to FIG. 11. FIG. 11(a) corresponds to FDMA. The analog signals of users 1-4 are subjected to frequency-division and transmitted over radio waves of different frequencies f1-f4. The signals of respective users 1-4 are separated by frequency filters.
FIG. 11(b) corresponds to TDMA. Digitized signals of respective users are transmitted over radio waves at different frequencies f1-f4, and time-divided for every prescribed period of time (time slot). The signals of respective users are separated by means of frequency filters and time-synchronization between a base station and each mobile terminal device of respective users.
In the SDMA system shown in FIG. 11(c), the data of a plurality of users are transmitted with one time slot of the same frequency divided spatially. In this SDMA, the signals of respective users are separated by means of frequency filters, time-synchronization between a base station and each mobile terminal device of respective users, and a mutual interference canceller such as an adaptive array.
FIG. 12 is a schematic block diagram showing a configuration of a transmission and reception system 2000 of a conventional base station for SDMA.
In the configuration shown in FIG. 12, n antennas #1-#n (n: natural number) are provided to establish identification between, for example, a user PS1 and a user PS2.
In a reception operation, the outputs of antennas are provided to an RF circuit 2101 to be amplified by reception amplifiers, and then frequency-converted by a local oscillation signal. The converted signals have the unnecessary frequency signal removed by filters, subjected to A/D conversion, and then applied to a digital signal processor 2102 as digital signals.
Digital signal processor 2102 includes a channel allocation reference calculator 2103, a channel allocating apparatus 2104, and an adaptive array 2100. Channel allocation reference calculator 2103 calculates in advance whether the signals from two users can be separated by the adaptive array. Based on the calculation result, channel allocating apparatus 2104 provides channel allocation information including user information, selecting frequency and time, to adaptive array 2100. Adaptive array 2100 applies a weighting operation in real time on the signals from antennas #1-#n based on the channel allocation information to separate only the signal of a particular user.
[Configuration of Adaptive Array Antenna]
FIG. 13 is a block diagram showing a configuration of a transmission and reception unit 2100a corresponding to one user in adaptive array 2100. The example of FIG. 13 has n input ports 2020-1 to 2020-n receiving the signals from antennas #1-#n, respectively, to extract the signal of the desired user from input signals of a plurality of users.
The signals input to input ports 2020-1 to 2020-n are applied via switch circuits 2010-1 to 2010-n to a weight vector calculator 2011 and multipliers 2012-1 to 2012-n. 
Weight vector calculator 2011 calculates weight vectors w1i-wni using input signals, a unique word signal that is the reference signal prestored in a memory 2014, and the output from an adder 2013. In the present specification, subscript “i” implies that the weight vector is employed for transmission/reception with the i-th user. Therefore, the unique word signal is a training signal for adaptive array processing.
Multipliers 2012-1 to 2012-n multiply the input signals from input ports 2020-1 to 2020-n by weight vectors w1i-wni, respectively. The multiplied result is applied to adder 2013. Adder 2013 adds the output signals from multipliers 2012-1 to 2012-n to output the added signals as a reception signal SRX(t). This reception signal SRX(t) is also provided to weight vector calculator 2011.
Transmission and reception unit 2100a further includes multipliers 2015-1 to 2015-n multiplying an output signal STX(t) of an adaptive array radio base station by respective weight vectors w1i-wni applied from weight vector calculator 2011. The outputs of multipliers 2015-1 to 2015-n are applied to switch circuits 2010-1 to 2010-n, respectively. Specifically, switch circuits 2010-1 to 2010-n provides the signals applied from input ports 2020-1 to 2020-n to a signal receiver unit 1R in a signal receiving mode, and provides the signal from a signal transmitter unit 1T to input/output ports 2020-1 to 2020-n. 
[Operating Mechanism of Adaptive Array]
The operating mechanism of transmission and reception unit 2100a of FIG. 13 will be described briefly here.
For the sake of simplifying the description with reference to the equations, it is assumed that there are four antenna elements, and two users PS effect communication at the same time. In such a case, signals applied to reception unit 1R from respective antennas are represented by the equations set forth below.RX1(t)=h11Srx1(t)+h12Srx2(t)+n1(t)  (1)RX2(t)=h21Srx1(t)+h22Srx2(t)+n2(t)  (2)RX3(t)=h31Srx1(t)+h32Srx2(t)+n3(t)  (3)RX4(t)=h41Srx1(t)+h42Srx2(t)+n4(t)  (4)
Signal RXj (t) represents a reception signal of the j-th (j=1, 2, 3, 4) antenna. Signal Srxi (t) represents a signal transmitted by the i-th (i=1, 2) user.
Coefficient hji represents the complex coefficient of a signal from the i-th user received at the j-th antenna, and nj (t) represents the noise included in the j-th reception signal.
The above equations (1)-(4) may be represented in vector form as follows:X(t)=H1Srx1(t)+H2Srx2(t)+N(t)  (5)X(t)=[RX1(t), RX2(t), . . . , RX4(t)]T  (6)Hi=[h1i, h2i, . . . , h4i]T, (i=1, 2)  (7)N(t)=[n1(t), n2(t), . . . , n4(t)]T  (8)
In equations (6)-(8), [ . . . ]T denotes the transposition of [ . . . ].
Here, X (t) represents the input signal vector, Hi the reception response vector of the i-th user, and N (t) a noise vector.
The adaptive array antenna outputs as a reception signal SRX (t) a synthesized signal obtained by multiplying the input signals from respective antennas by respective weight coefficients w1i-w4i, as shown in FIG. 13.
Given these preliminaries, the operation of an adaptive array in the case of extracting a signal Srx1 (t) transmitted by the first user, for example, is set forth below.
Output signal y1 (t) of adaptive array 2100 can be represented by the following equations by multiplying input signal vector X(t) by weight vector W1.y1(t)=X(t)W1T  (9)W1=[w11,w21,w31,w41]T  (10)
In other words, weight vector W1 is a vector with the weight coefficients wj1 (j=1, 2, 3, 4) to be multiplied by the j-th input signals RXj (t) as elements.
Substituting input signal vector X (t) represented by equation (5) into y1 (t) represented by equation (9) yields:y1(t)=H1W1TSrx1(t)+H2W1TSrx2(t)+N(t)W1T  (11)
By a well known method, weight vector w1 is sequentially controlled by weight vector calculator 2011 so as to satisfy the following simultaneous equations when adaptive array 2100 operates in an ideal situation.H1W1T=1  (12)H2W1T=0  (13)
If weight vector W1 is perfectly controlled so as to satisfy equations (12) and (13), output signal y1 (t) from adaptive array 2100 is eventually represented by the following equations.y1(t)=Srx1(t)+N1(t)  (14)N1(t)=n1(t)w11+n2(t)w21+n3(t)w31+n4(t)w41  (15)
Specifically, signal Srx1 (t) emitted from the first of the two users will be obtained for output signal y1 (t).
In FIG. 13, input signal STX (t) for adaptive array 2100 is applied to transmitter unit 1T in adaptive array 2100 to be applied to respective one inputs of multipliers 2015-1, 2015-2, 2015-3, . . . , 2015-n. To the other inputs of these multipliers, weight vectors w1i, w2i, w3i, . . . , wni calculated by weight vector calculator 2011 based on reception signals described above are copied and applied.
The input signals weighted by these multipliers are delivered to corresponding antennas #1, #2, #3, . . . , #n via corresponding switches 2010-1, 2010-2, 2010-3, . . . , 2010-n for transmission.
Identification of users PS1 and PS2 is made as set forth below. A radio wave signal of a cellular phone is transmitted in frame form. The radio wave signal of a cellular phone is mainly composed of a preamble formed of a signal series known to a radio base station, and data (voice and the like) formed of a signal series unknown to the radio base station.
The preamble signal series includes a signal stream of information (unique word signal) to identify whether the current user is the appropriate user to converse for the radio base station. Weight vector calculator 2011 of adaptive array radio base station 1 compares the unique word signal output from memory 2014 with the received signal series to conduct weight vector control (determine a weight coefficient) so as to extract the signal expected to include the signal series corresponding to user PS1.
The above description is based on a configuration in which the weight vector of the reception mode is copied to form the directivity of a transmission signal in a signal transmission mode. Alternatively, the weight vector of the reception mode can be corrected to be used as the weight vector for transmission taking into account the travel speed or the like of the terminal device in a transmission mode.
As a communication system of high usage efficiency of frequency, the orthogonal frequency division multiplexing (OFDM) scheme is known.
The OFDM scheme is one type of multicarrier modulation of spreading data of one channel into a plurality of carrier waves for modulation. In the OFDM scheme, the frequency spectrum of the signal employed in communication is substantially rectangular.
FIG. 14 shows the extraction of three carriers (carrier waves) of the frequency spectrum of a plurality of carriers employed in the OFDM scheme.
Attention is focused on the spectrum of one carrier wave shown in FIG. 14. In the OFDM scheme, the frequency interval of a plurality of carrier waves is set so that the zero point of the spectrum of this one carrier wave matches the frequency of an adjacent carrier wave. In other words, each carrier wave is arranged at a frequency avoiding mutual interference, and each carrier wave is orthogonal to each other.
The interval Δf of the frequency of each carrier wave is expressed by the following equation, where Ts is the duration of one symbol of transmitted data.Δf=1/Ts×n (n: natural number)
FIG. 15 represents the waveform of the symbol transmitted in accordance with the OFDM scheme.
As a result of combining the waveforms of i=1 to i=N carrier waves, i.e., a total of N carrier waves, a signal represented by the bottom most waveform in FIG. 15 is employed as the transmission symbol of OFDM.
In order to obtain each carrier component in the modulation of the OFDM scheme, inverse discrete Fourier transform is carried out on the baseband signal. Correspondingly, in the demodulation process of a reception wave, discrete Fourier transform is applied on the reception signal through the algorithm of the so-called Fast Fourier Transform (FFT).
In the OFDM signal waveform in FIG. 15, a “guard interval” is provided before the valid symbol period. Such a guard interval has a portion of the valid symbol waveform, for example a signal of a predetermined time Tg at the tail of the valid symbol waveform, copied and added.
The guard interval is provided as countermeasures against an interfering wave caused by multipath interference.
In the case where a desired wave and an interfering wave arriving behind time are combined to form a reception signal, the effect of the interfering wave is limited within the guard interval period if the delay time of the interfering wave is within the time set as the guard interval. By setting the guard interval period longer than the expected delay time of an interfering wave, demodulation can be performed with the effect of an interfering wave removed.
FIG. 16 is a schematic diagram to describe a demodulation operation when such a desired wave and interfering wave are received.
In demodulation according to the OFDM scheme, a time window termed “FFT window” is provided in each symbol period, as shown in FIG. 16. This time window denotes the section corresponding to the process of cutting out only the valid symbol section from the received OFDM transmission symbol. The FFT window is set equal to the valid symbol period length Ts. The guard interval period is set longer than the delay time of an interfering wave, as mentioned above. Accordingly, the orthogonality of each carrier wave of a reception wave can be maintained even if there is an interfering wave since a signal present in the guard interval period is a signal in the same valid symbol. Thus, demodulation with the effect of such an interfering wave removed can be carried out at the receiving side.
It is expected that a higher communication quality and a reception scheme of higher usage efficiency of radio frequency can be realized by the combination of the above-described adaptive array scheme and OFDM scheme.
However, the mere combination of the two schemes will pose problems set forth below.
[Problem in Configuration of Operating Adaptive Array Differing for Every Carrier]
An example of a first configuration for OFDM transmission using an adaptive array will be described hereinafter.
By such a configuration, the above-described multiple access of the SDMA scheme can be established by application of adaptive array technique.
FIG. 17 is a schematic block diagram to describe a configuration of such an adaptive array base station 3000.
Referring to FIG. 17, it is assumed that adaptive array base station 3000 conducts transmission and reception using an adaptive array antenna including four antennas #1-#4, for the sake of simplification. In FIG. 17, description is based on a configuration directed to reception in accordance with the configuration of an adaptive array base station.
Referring to FIG. 17, adaptive array base station 3000 includes an A/D converter 3010 receiving signals from adaptive array antennas #1-#4 to carry out detection and analog-digital conversion, and an FFT unit 3020 applying fast Fourier transform on a received digital signal from A/D converter 3010 to separate the signal for each carrier wave.
In the present description, the signal from the i-th antenna for the first carrier among the signals output from FFT unit 3020 is represented as signal f1, i (1, i: natural number).
Adaptive array base station 3000 further includes N (N: total number of carriers) adaptive array blocks 3030.1-3030.N provided for each carrier. Each adaptive array block receives the component of a corresponding carrier obtained by applying Fourier transform on the signal from antennas #1-#4 through FFT unit 3020 to carry out adaptive array processing.
It is to be noted that only adaptive array block 3030.1 for the first carrier is depicted in FIG. 17.
Adaptive array block 3030.1 includes, likewise the adaptive array base station shown in FIG. 13, a reception weight vector calculator 3041 receiving signals f1, 1-f1, 4 to calculate a reception weight vector, multipliers 3042-1 to 3042-4 receiving signals f1, 1 to f1, 4 at respective one inputs and the reception weight vector from reception weight vector calculator 3041 at respective other inputs, an adder 3043 to receive and combine the outputs of multipliers 3042-1 to 3042-4, and a memory 3044 to prestore a unique word signal (reference signal) used in the calculation of adaptive array processing by reception weight vector calculator 3041. Adder 3043 outputs a desired signal S1 (t) for carrier 1. This desired signal S1 (t) is also applied to reception weight vector calculator 3041.
By such a configuration, the signal from a desired user can be separated for each carrier from a signal transmitted by the OFDM transmission scheme by adaptive array processing for reception.
In the configuration of such an adaptive array base station 3000, the following problems are noted.
As described above, the signal of one channel is spread into a plurality of carriers for transmission in the OFDM scheme.
Therefore, the number of symbols of a reference signal included for each carrier is often not sufficient for the signals transmitted through the OFDM scheme. For example, in “multimedia mobile access communication systems (MMAC) recommended by the Ministry of Public Management or the like, two symbols are defined for the reference signal for each OFDM carrier (subcarrier).
In this case, it will be difficult to converge the weight based on the configuration of adaptive array base station 3000 shown in FIG. 17. There was a problem that directivity of favorable accuracy could not be established.
Furthermore, the configuration of adaptive array base station 3000 shown in FIG. 17 has problem set forth below.
FIG. 18 is a schematic diagram representing the timing of a signal received at adaptive array base station 3000 of FIG. 17.
In FIG. 18, the section labeled “G” represents the above-described guard interval period of a reception signal.
The primary desired wave is generally the first signal arriving at the base station. The first arriving signal is referred to as “head arriving signal” hereinafter.
With respect to this head arriving signal, a signal arriving in delay within the guard interval period is called a “short delayed signal” whereas a signal arriving in delay for at least the guard interval period from the head arriving signal is referred to as a “long delayed signal”, under the influence of multipath. The route through which each of a head arriving signal, a short delayed signal, and a long delayed signal is transmitted is referred to as a “path”.
In FIG. 18, the signal sampling timing in adaptive array block 3030.1 is denoted with an arrow.
Since adaptive array processing is carried out on a signal that has been divided for each carrier in adaptive array base station 3000, the sampling timing of a signal is to be set at a time interval sufficient for extracting a signal waveform for each carrier.
By adaptive array processing, a long delayed signal as shown in FIG. 18 can be removed.
The bandwidth of a band-divided carrier is so narrow that a short delayed signal cannot be separated. Therefore, processing is carried out with the head arriving signal and the short delayed signal regarded as the same signal in adaptive array processing.
FIG. 19 shows the intensity distribution of signals corresponding to respective carriers after passing through such an adaptive array.
In each of frequencies f1-fN of carriers in FIG. 19, the spectrum of the head arriving signal (head wave) and the spectrum of a short delayed signal (short delayed wave) appear to be the same signal after the adaptive array processing, as mentioned above. However, since the band for the entire carriers is extremely wide, there may be the case where the head wave and the short delayed wave are opposite in phase in the carrier indicated by the arrow in FIG. 19.
FIG. 20 represents the intensity distribution when the signals for respective carriers are combined in the case of FIG. 19.
If adaptive array reception is conducted using a reference signal timed to the head wave, only a signal of small level can be extracted for the carrier of a frequency having the head signal and the short delayed signal in opposite phase. In other words, if adaptive array reception is conducted for each carrier, only a signal of an extremely low signal level can be extracted for a carrier of a frequency that has the head wave and the short delayed wave in opposite phase, as shown in FIG. 19.
Since sufficient signal transmission cannot be conducted for the carrier indicated by the arrow in FIG. 19, a redundancy code must be used or control must be provided to communicate without using this carrier. The latter is equivalent to remove as an unnecessary signal a signal originally arriving at the base station as a short delayed signal. This will lead to degradation in reception sensitivity
Summarizing, there is a problem that is difficult to ensure a sufficient reference signal required for directional control of high accuracy in the configuration of operating an adaptive array differing for each carrier as shown in FIG. 17.
There is also a problem that the reception sensitivity is degraded since multipath signals within the guard interval cannot be combined in maximum ratio.
In other words, since a signal of a delay time within the guard interval (short delay component) has high correlation with the head signal, a short delay component will be included in the array combined output if combining based on the adaptive array is carried out using a reference signal timed to the head signal. However, in the case where a plurality of carriers employed in communication are distributed over an extremely wide band in the OFDN scheme, there may be a case where the head wave and the short delayed wave are opposite in phase depending upon the carrier. In such a case, there will be a problem that combination at the maximum ratio is not conducted when viewed over the entire carrier.
[Problems Based on a Configuration of Adaptive Array Operation with Weight to Entire Carrier]
In view of the above-described problems in the configuration of adaptive array base station 3000, an approach of another configuration may be considered, conducting adaptive array processing on a signal prior to band-division by an FFT process.
FIG. 21 is a schematic block diagram to describe a configuration of an adaptive array base station 4000 operating an adaptive array, calculating a common weight for all the carriers.
Referring to FIG. 21, adaptive array base station 4000 includes, likewise adaptive array base station 3000 of FIG. 17, an A/D converter 4010 applying detection and analog-digital conversion on signals received from four antennas #1-#4, a reception weight vector calculator 4041 receiving outputs of A/D converter 4010 to calculate reception weight vectors for signals of respective antennas, multipliers 4042-1 to 4042-4 receiving signals from array antennas at respective one inputs, and receiving weight vectors from reception weight vector calculator 4041 at respective other inputs, an adder 4043 to receive and combine outputs from multipliers 4024-1 to 4042-4, a memory 4044 to prestore a reference signal employed in calculating weight vectors by reception weight vector calculator 4041, and an FFT unit 4050 applying fast Fourier transform processing on a received output from adder 4043 for separating into signals S1(t)-SN(t) of the desired waves for respective carriers. The output from adder 4043 is applied to reception weight vector calculator 4041 to be used in the calculation of a reception weight vector.
FIG. 22 is a schematic diagram to describe an operation of adaptive array base station 4000 of FIG. 21.
In FIG. 22, “G” denotes a guard interval period. For the purpose of applying adaptive array processing on a signal not yet subjected to band-division, the sampling timing of, for example, reception weight vector calculator 4041 in the adaptive array must be set shorter than that for a signal subjected to band division as shown in FIG. 18.
A long delayed signal can similarly be removed by adaptive array processing through an adaptive array block.
The signal applied to an adaptive array block has an extremely wide band since it is not band-divided. In other words, a head arriving signal and a short delayed signal will be recognized as completely different signals at reception weight vector calculator 4041. Therefore, such short delayed signals will be removed by adaptive array processing.
This operation is disadvantages in that, although the short delayed signal per se is a desired wave whose property may be improved if used effectively, such a short delayed signal will be removed by the adaptive array processing, resulting in the problem of degradation in communication quality.
Furthermore, since a short delayed signal will be regarded as an interfering signal, it will look as if a large number of interfering waves are arriving when viewed on part of adaptive array base station 4000. If directivity is established by the adaptive array in order to remove such signals, there is a possibility of no degree of freedom of the antenna left.
If there is no degree of freedom of antenna left, the gain towards the direction of a desired wave will be degraded, or the interference may not be completely removed since it will look as if there are interference exceeding the antenna degree of freedom.
The present invention is directed to overcome the above-described problems. An object of the present invention is to provide an adaptive array base station that can combine at the maximum ratio the multipath signals within a guard interval to improve reception sensitivity even in the case of adaptive array reception with respect to the OFDM transmission scheme.
Another object of the present invention is to provide an adaptive array base station that can maintain the interference suppression performance without consuming the antenna degree of freedom in combining multipath signals within a guard interval period.